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Split the fingerprint's identity out, and tsc=reliable #2

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Merged
aledbf merged 2 commits into from
Sep 8, 2026
Merged

Split the fingerprint's identity out, and tsc=reliable #2

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121b059
Split the fingerprint's identity out, so a memo can key on all of it
aledbf Sep 8, 2026
4a006f3
tsc=reliable, which was left behind when the command line moved here
aledbf Sep 8, 2026
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7 changes: 6 additions & 1 deletion machine/cmdline.go
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Original file line number Diff line number Diff line change
Expand Up @@ -134,8 +134,13 @@ func (c Cmdline) String() string {
// Timing shortcuts a KVM guest can take:
// no_timer_check skip the boot-time timer IRQ delivery probe,
// which exists for hardware that misroutes it.
// tsc=reliable trust the TSC and skip the clocksource watchdog.
// The watchdog exists for silicon whose TSC drifts
// or stops; this machine's TSC is the host's, which
// KVM advertises as invariant, and the guest is
// told so through CPUID.
// rcupdate.rcu_expedited=1 expedite RCU grace periods during boot.
parts = append(parts, "no_timer_check", "rcupdate.rcu_expedited=1")
parts = append(parts, "no_timer_check", "tsc=reliable", "rcupdate.rcu_expedited=1")

parts = append(parts, c.Extra...)

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30 changes: 27 additions & 3 deletions machine/machine.go
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Expand Up @@ -642,8 +642,6 @@ func (s Spec) Args() ([]string, error) {
// every VM restored from one have their memory in a file whatever the spec being
// asked was configured with.
func (s Spec) Fingerprint() (string, error) {
shape := s.TemplateShape()

h := sha256.New()

// Length-prefixed, so that no two different machines can produce the same
Expand Down Expand Up @@ -675,6 +673,32 @@ func (s Spec) Fingerprint() (string, error) {
}
write(f.name, sum)
}
ident, err := s.Identity()
if err != nil {
return "", err
}
write("identity", ident)

return hex.EncodeToString(h.Sum(nil)), nil
}

// Identity is everything the fingerprint hashes except the contents of those
// three files: the machine's shape, its device topology, and the host's own CPU
// when the guest is being shown it.
//
// It is separate because reading the three files is the expensive half — 76 MB
// of SHA-256, 29 ms on a machine measured — and a caller that memoises that half
// needs the other half whole to key the memo on. Under-keying it is the failure
// that has no symptom: a stale fingerprint is a template that matches a machine
// it does not describe, and a restore into it is undefined rather than an error.
// So there is no list here for a caller to keep in step; there is this.
func (s Spec) Identity() (string, error) {
shape := s.TemplateShape()

var b strings.Builder
write := func(key, value string) {
_, _ = fmt.Fprintf(&b, "%s=%d:%s\n", key, len(value), value)
}
write("machine", shape.Machine)
write("cpu", shape.CPU)
write("smp", shape.SMP)
Expand Down Expand Up @@ -703,7 +727,7 @@ func (s Spec) Fingerprint() (string, error) {
write("host-cpu", cpu)
}

return hex.EncodeToString(h.Sum(nil)), nil
return b.String(), nil
}

// topology is the machine's device list — which models, at which slots — with
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